High intensity discharge tungsten halogen and metal halide lamps enclosed in quartz housings emit high levels of ultraviolet radiation which are not sufficiently blocked by their quartz housings. It is necessary to apply a suitable UV absorbing coating to absorb wavelengths less than 320 nanometers in order to protect personnel from detrimental health problems and interior furnishings and lamp fixtures. Sol gel coatings comprising colloidal cerium oxide particles in an inorganic oxide matrix formed by the hydrolysis and condensation of an alkoxysilane and/or other metal alkoxide have been developed for coating plastic substrates. These coatings have an absorbance of 0.1 to 0.25 per micron of coating thickness and require a thickness of several microns to assure an absorbance greater than 1, the minimum considered needed to prevent damage from exposure to dangerous ultraviolet radiation if such protective coatings are absent.
The techniques developed for coating plastic substrates with cerium oxide containing coatings of necessity are limited to a maximum curing temperature to cure the coated plastic substrates to insure the integrity of the substrate. Thus, prior art curing steps were performed at maximum temperatures of 120.degree. C. to 130.degree. C. When techniques developed for coating plastic substrates with these cerium oxide coating compositions were used on glass or quartz substrates cured under conditions suitable for plastic substrates, the absorbance obtained was on the order of magnitude developed for coated plastic substrates. However, coatings applied to glass or quartz substrates crazed and/or peeled when heated to elevated temperatures beyond the curing temperatures used for coating plastic substrates.
High intensity lamps enclosed in quartz housing cause the coatings applied by the techniques developed for coating plastic substrates to craze when the coated quartz housings are heated to temperatures exceeding 1000.degree. C., such as those approaching 1100.degree. C., during use. Also, when such lamps are not used continuously, they are subjected to temperature cycles that vary between ambient temperature and the aforesaid elevated temperatures. These wide temperature cycles increase the chance of the coating to craze and peel. A technique was needed to enable those skilled in the art of coating glass and quartz to obtain the benefits of the well known ultraviolet absorbance of coatings comprising colloidal cerium oxide particles essentially uniformly diffused through a silica-based matrix while avoiding the problem of crazed coatings associated with exposure of the coated article to elevated temperatures and of haze problems that occur due to agglomeration of the colloidal cerium oxide particles when the latter are not passivated efficiently in the oxide matrix.
U.S. Pat. No. 4,799,963 to Basil et al. discloses ultraviolet radiation resistant coatings comprising colloidal cerium oxide. Coatings that protect plastic substrates such as polycarbonate from damage caused by ultraviolet radiation are formed from aqueous sols containing colloidal cerium oxide and a glassy network of materials derived from metal alkoxides such as alkoxysilanes and other metal alkoxides. The alkoxysilane may be an organoalkoxysilane, such as an alkylalkoxysilane or organofunctional alkoxysilane. The alkoxide may contain alkyl or aryl groups and may be in higher condensed form so long as hydrolyzable alkoxide groups remain. The alkoxide may be added directly to an aqueous cerium oxide sol, with or without additional solvent such as alcohol. Hydrolysis and condensation polymerization occur in situ. Alternatively, the alkoxide may be partially or fully hydrolyzed, and condensed to some extent prior to combination with the cerium oxide sol.
The resulting silane/ceria sol may be used as a coating composition for either a primer or protective overcoat on a substrate or coated substrate. Or, the silane/ceria sol may be added to other coating compositions to increase their resistance to ultraviolet radiation. In either case, the coating compositions may be applied by any conventional technique, such as spraying, dipping or flow coating. The composition dries and cures to form a uniform durable coating with good adhesion to plastic substrates such as polycarbonate. The coating protects the substrate from ultraviolet radiation by providing a strong, broad absorption band in the 200 to 320 nanometer range.
In a preferred embodiment of Basil, an alkoxide is partially hydrolyzed before adding an aqueous sol of colloidal cerium oxide. Preferably, the alkoxide is an alkoxysilane of the general formula R.sub.x Si(OR').sub.4-x wherein R is an organic radical, R' is selected from the group consisting of low molecular weight alkyl radicals, and x is less than 4 and may be zero. The organic radical of R is preferably alkyl, vinyl, methoxyethyl, phenyl, .gamma.-glycidoxypropyl, or .gamma.-methacryloxypropyl. The alkoxide hydrolyzes according to the general reaction EQU R.sub.x Si(OR').sub.4-x +.sub.y H2O.fwdarw.R.sub.x Si(OR').sub.4-x-y (OH).sub.y +.sub.y R'OH
Condensation of the hydrolyzed alkoxide proceeds according to the general reactions ##STR1## Further hydrolysis and condensation follow.
The pH and/or degree of condensation of the alkoxides may be adjusted, if necessary, to prevent haze or precipitation upon mixing with the ceria sol. The cerium oxide sol preferably comprises about 10 to 30 percent by weight colloidal cerium oxide in water, with the colloidal cerium oxide particle size sufficiently small to minimize scattering of visible light, preferably less than 30 to 40 nanometers, most preferably less than 10 nanometers. Alkoxides of titanium and/or zirconium may also be included in compositions of Basil, as well as colloidal silica for abrasion resistance. The ultraviolet radiation protection provided by the cerium oxide containing coating of Basil may be determined by measuring the UV absorbance spectrum of the coating applied on a quartz substrate.
The Basil patent includes two examples of primed polycarbonate substrates and two examples of quartz substrates coated by applying aqueous sols containing colloidal cerium oxide and a network forming metal alkoxide to measure the absorbance of the coatings. The sols formed coatings containing on the order of 7 weight percent and 4 to 5.5 mole percent of cerium oxide based on the total metal oxide of the coatings. The coatings were cured at temperatures ranging up to on the order of 120.degree. C. A coating thickness of 6.5 microns was reported. The absorbance of the resulting coatings was on the order of 0.1 to 0.2 per micron of coating thickness.
The coatings containing colloidal cerium oxide reduced the tendency of polycarbonate articles of Examples I and II of the Basil patent to become yellow and embrittled upon exposure to UV radiation. The coatings applied to quartz improved the UV absorbance of the quartz articles of Examples III and IV. There is no report of testing these coated articles at elevated temperatures to determine the resistance of crazing of these coatings at such elevated temperatures.
U.S. patent application Ser. No. 546,484, filed Jun. 29, 1990, by Lin et al. discloses and claims an abrasion resistant coating of organic hybrid polymers of alkoxysilanes containing colloidal cerium oxide for ultraviolet absorption properties that are formed from siloxane compositions that also contain polyvinyl pyrrolidone and sodium acetate applied to a rigid transparent plastic substrate treated with an acrylic primer by dip coating for five minutes at room temperature and cured at 120.degree. C. to develop a cured coating having a preferable thickness of about four microns. The polyvinyl pyrrolidone is provided to optimize film formation and the sodium acetate is added to promote complete cure of the siloxane polymer preferably in the range of 80.degree. C.
From the foregoing, it is evident that prior to the present invention, it was known to coat plastic and quartz substrates with sol-gel compositions containing colloidal cerium oxide particles in an inorganic oxide matrix formed by the hydrolysis and condensation of an alkoxysilane and/or other metal alkoxide to improve the UV absorbance of the coated substrates and reduce exposure of the substrates to UV radiation. Curing was accomplished at temperatures sufficient to bond the coating to the substrate without harm to the plastic substrate. The prior art did not appreciate that the low curing temperatures used after coating plastic substrates with such sol-gel compositions would not be suitable to insure craze-free coatings on quartz or glass substrates when quartz or glass substrates are exposed to elevated temperatures after being cured at the low curing temperature suitable for plastic substrates. Also, the prior art did not appreciate that the prior art concentrations of colloidal cerium oxide particles in sol-gel compositions necessitated coatings of such thickness required for a desired high level of absorbance that obtaining a crack-free, non-peeling coating was precluded.
The prior art was not aware that the higher curing temperatures after coating would reduce and even eliminate crazing of glass or quartz substrates coated with sol-gel compositions containing colloidal cerium oxide particles when the coated substrates are exposed to elevated temperatures and/or that using coating compositions having higher concentrations of colloidal cerium oxide than those used in the prior art would provide coatings having greater absorbance per unit thickness than those obtained from the prior art techniques, thereby obtaining better bonding of a sol-gel coating to a glass or quartz substrate for any desired absorbance.